Electron beam lithography machines are primarily used for, in particular, writing patterns, such as electrical circuits, on substrates. In such a process, a suitably prepared surface of a substrate supported on a movable stage is exposed to a focussed electron beam, in particular a writing spot or probe of the beam, which traces an intended pattern on the surface and generates the pattern by, for example, corresponding influence on a resist coating of the surface. The resolution of the beam is, for practical purposes, unlimited. The ultimate resolution limit of focussed electron beams is encountered in the realm of transmission electron microscopes, which operate at atomic dimensions of 0.1 nm. This is two to four orders of magnitude smaller than practical dimensions for lithography machines. However, with respect to throughput, electron beam lithography has been too slow for applications such as mainstream silicon VLSI manufacturing. There are two reasons for this:
i) Electron beam exposure is a serial process, in which a single beam scans the pattern area to expose pattern features in sequence. By contrast, in optical lithography an entire pattern can be exposed at the same time.
ii) The usable current in probe-forming electron beam systems is limited by Coulomb interaction in the beam path, whereby increase in the current causes the writing spot to blur and thus degrades resolution.
All electron beam lithography machines commonly in use exhibit these characteristics. The serial nature of the exposure process can be improved by exposing multiple pixels in parallel. A Gaussian beam system exposes merely one pixel at a time and thus is inherently serial. A variable shaped beam system exposes, for example, between 16 and 256 pixels simultaneously and, by virtue of the pixel parallelism, is necessarily faster than a Gaussian system. Similarly, cell projection and electron beam projection are faster than variable shaped beam systems, because they utilise a higher degree of pixel parallelism.
Throughput can be characterised as pattern-writing speed, which is defined as, for example, maximum area swept out per unit time by the writing probe. One possible method of increasing throughput is to increase the writing current. However, blurring of the writing spot, or loss of resolution, caused by the Coulomb interaction becomes more pronounced as the writing current is increased. In order to obtain acceptable clarity of the written pattern it is necessary for the lateral blurring of the spot to be a reasonably small fraction of the minimum size of the features of the pattern. Above a limiting value of the writing current the blurring becomes unacceptably pronounced for the size of pattern features desired. The smaller the pattern features, the less the limiting value of the writing current. This results in a compromise between resolution and throughput.
In order to take full advantage of pixel parallelism for a variable shaped beam it would be necessary to increase the writing current by a factor equal to the pixel parallelism. This is not practical, as the Coulomb interaction would unacceptably degrade resolution. All probe-forming electron beam systems are limited with respect to usable writing current by the Coulomb interaction, with systems utilising high pixel parallelism being somewhat faster.
In addition to the constraints on throughput discussed in the foregoing, another limitation arises from the requirement for every electron beam system to perform various functions during which no actual writing takes place. These functions include loading and pumping the substrate, moving the support stage to new locations, deflecting the beam to a new scanning position and calibrating the system. Actions of these kinds result in an overhead time which detracts from throughput. In many cases the overhead time represents a significant fraction of the total time needed to process a substrate. In these cases it would be desirable to reduce the overhead time to the minimum possible.
A significant overhead is represented by the time required for the stage to move to a new location in order to accomplish a large area exposure. Whereas many of the other overheads are electrical in origin, the stage motion is mechanical and results in an overhead which is inherently larger than an electrical overhead due to the inertia of the usual relatively substantial stage construction. In the case of a pattern with relatively few or widely spaced features, the overhead times associated with stage motion to locate the feature areas in a writing zone can form the most significant limitation on throughput.
In order to make full use of the high resolution capability of an electron beam it is necessary to ensure accurate placement of pattern features to within a small fraction of the minimum pattern feature size, i.e. to provide precise beam deflection to scan the pattern features. Beam deflection is usually controlled by a beam deflector powered by electronic amplifiers which provide a trade-off between noise and bandwidth. The lower the bandwidth, the less the noise. However, a high throughput requires a high bandwidth to achieve the desired speed to minimise the overhead time associated with the beam deflection. A high bandwidth results in a greater degree of noise, which degrades the placement accuracy of pattern features. This area, in particular the electronic noise connected with the beam deflection, represents another conflict between the requirements of resolution and throughput.
All of the factors mentioned in the foregoing lead to the situation that it is possible to obtain high resolution or high throughput, but not both at the same time. Known electronic beam systems of given construction are, by virtue of design and system configuration, useful for either high resolution or high throughput. High resolution usually requires a finely focused beam, high electronic precision and high mechanical stability. High throughput requires high writing current, high-speed electronics and minimal overhead times. Existing electron beam systems cannot be configured to achieve all these attributes and would require fundamental adaptation of basic components to change from high resolution to high throughput.